SAS (Serial Attached SCSI) has been known as one of data transfer methods between a host computer and a peripheral apparatus. According to SAS, devices connected under the SAS standard (hereinafter called a “SAS device”) are concatenated through one or a plurality of bus connection adjusting units (hereinafter, called expanders) which are responsible for adjustment of bus connection.
When an access conflict occurs between SAS devices, an expander selects an SAS device having the longest AWT (Arbitration Wait Time) with priority under the SAS protocol for access leveling.
The AWT refers to a waiting time from the time when an SAS device issues a bus connection request (OAF: OPEN Address Frame) to the time when the bus connection is established.
When expanders are provided in a hierarchical structure, the OAFs selected by a lower expander are aggregated to a higher expander. The higher expander selects the OAF having the longest AWT with priority.
However, OAFs do not reach the higher expander at a predetermined time. If OAFs reach at different times, an OAF having a longer AWT than the AWT of the OAF selected by the expander may reach the expander later. Even in this case, the expander may not be able to select the OAF having reached later, and the AWT of the OAF having reached later may possibly further increase.
Thus, the OAF having a longer AWT finally times out, preventing the establishment of the bus connection.
Having described a system applying the SAS standard, the similar problems may occur in systems applying other standards for control over priority in and leveling of access processing.